Many networks today have a layered network architecture modeled on the Open Systems Interconnection (OSI) reference model developed by the International Standards Organization (ISO). The OSI reference model has seven protocol layers viz. an application layer, a presentation layer, a session layer, a transport layer, a network layer, a data link layer, and a physical layer.
The lowest layer is the physical layer and its function is to transfer bits over a communications medium. The next layer is the data link layer whose function is to partition input data into data frames and to sequentially transmit the frames over the physical layer. Each data frame includes a header which contains control and sequence information for the frames.
The interface between the data link layer and the physical layer includes a medium access control device, called a MAC device, and a physical layer device, called a PHY device, which controls the serial transmission of bits over the communications medium. The purpose of the MAC device and the PHY device is to ensure two network stations communicate using the correct frame formatting protocol.
In Wireless Local Area Networks (WLANs) a radio is the PHY device and free space is the physical communications medium. The Institute of Electrical and Electronic Engineers Standard IEEE 802.11 is a standard for WLAN communications and defines a communications protocol between a MAC device and a radio PHY device.
In wired Local Area Networks (LANs) the PHY device includes a port such as a Universal Serial Bus (USB) bus port and a physical cable is the physical communications medium.
Advantageously, notebook computers should support communications over a wireless medium and a wired medium. Thus, a user could take advantage of e.g. a 100 MB LAN connection when in an office or a 10 MB WLAN connection when away from the office. Due to the small form factors of notebook computers, there is a need to combine MAC and PHY devices to produce a device that supports both communications over a wireless medium and over a wire medium.